A while back I purchased a NUC6i7KYK to use with a dual display port Thunderbolt adapter (StarTech TB32DP2) and two Dell U3415W monitors . This didn't work (only one monitor works at a time this way). As a workaround I had to use one of the other ports (mini DP or HDMI).

Intel tech support claimed that it isn't possible (though this limitation is not documented as far as I know). Any status on having this feature added so that we can use Thunderbolt adapters and docks that are coming onto the market?

In regard to your inquiry, at this point there is no official information directly from Intel® stating that this feature will be added in the future.

On the following link you will be able to confirm that in fact when you use an adapter from thunderbolt to Display port, it will work just for one monitor, please look where it says “What's supported via Thunderbolt™ 3 on an Intel® NUC Kit NUC6i7KYK?” second line:

In regard to your inquiry, there is not an official document stating that the feature will be added in the future or not, let me apologize for that, but if by any chance Intel® decides to add it, you will be able to see that information in our official web site:

Hey Kielby, the reason why it won't work is that due to the size and the layout of the NUC we were only able to route 1 signal to the Thunderbolt controller. Which means that when you use a Type C/Thunderbolt adapter to a splitter it is only able to provide 1 signal (see attached). However, if you were to have monitors that you could daisy chain, it should work if you go out of the first monitor in to the second monitor. I'll try it tomorrow to confirm.